The present invention relates generally to rotary valves and, more particularly, to fluid flow control plug valve closures.
Concentric plug-type rotary valves for fluid flow control typically include, as shown in FIG. 1A and 2, valve body or housing 100 disposed between multiple sections of pipe 102 in the fluid flow path. This valve housing contains a valve closure member 20 which is pivotal about axis 108, generally perpendicular (out of the plane of FIG. 1) to the primary direction of fluid flow along axis 104, and slides past surface 30 which defines port 10 in the fluid flow path to restrict the rate of fluid flow through that port. Surface 30 and valve closure member 20 are typically machined so as to have a common radius of curvature in order to form mating seats or surfaces which provide fluid sealing under various temperatures and pressures.
Since closure member 20 and/or surface 30 of the concentric valve discussed above would wear with contact, this type of valve has been generally used for flow control and not for complete shut off. Where complete shut off has been desired, prior rotating valves often have included eccentric valve closure members such as 20' which although pivotable about an axis 108, are shaped as segments of arcs with radius R swung about offset axis 110, as shown in FIG. 1B. Such arrangements attempt to permit the valve closure member to move toward eccentric valve seat 30' with minimum sliding contact therewith until the moment of complete closure. These offset axis valves are commonly referred to as "eccentric" valves. An example of such eccentric valve structure is shown in U.S. Pat. No. 2,803,426, issued to David DeZurik and assigned to the assignee of the present invention. As best illustrated in FIGS. 5 and 6 of that patent, the pivotal axis of the valve closure member lies along the centerline axis of fluid flow. The valve seat is machined to have inward sealing extensions which contact the face of the valve closure member. The cross-sectional curvature of the surface of these sealing extensions is eccentric with respect to the pivotal axis of the valve closure member, such that the constant radius of the sealing surface of the valve seat is swung from a point offset from the pivotal axis shown in FIG. 5 of that patent. This results, as seen in the Figures, in initial seat edge 30 projecting inwardly less than terminal seat edge 31.
To achieve proper sealing against fluid leakage through the port in eccentric valves, the valve closure member and the valve seat must be precisely aligned when brought together by rotation of the valve closure member. Machining and assembling these elements to within close manufacturing tolerances can become relatively expensive. To alleviate this problem, some prior art devices included spring arrangements to adjust the position of the valve elements with respect to each other within the valve body or housing. Unfortunately, such arrangements are rather complicated to assemble and do not compensate fully for dimensional variation with certain temperature and pressure changes nor function properly with fluids containing entrained particulate matter. Other problems are encountered with prior eccentric valve devices during overclosure, i.e., when the valve closure member is moved into too tight a contact with the valve seat. To prevent galling of the metal valve faces and yet provide tight shut-off without the use of sealing lubricants, it has been suggested to cover the valve closure member or the seat surface or both with an elastomeric material. However, when the valve seat and the valve closure member have sealing faces in the form of simple cylinders of circular arcs, the effectiveness of fluid sealing may vary over the perimeter of the port valve seat as the valve closure member moves from initial contact with the valve seat to closed and then to an overclosed position. FIGS. 3, 4 and 5 show enlarged partial cross-sectional views of a typical elastomer-covered eccentric valve closure of the offset pivotal axis type at three different closed valve positions which illustrate these situations. While eccentric valves of the type shown in the above-mentioned patent are not also shown explicitly in the present application, it will be readily understood by those skilled in the art that similar overclosure problems can arise therein also.
FIG. 3 shows, in a plane viewed parallel to the fluid flow taken along line I--I of FIG. 2, a portion of valve housing 100 containing port 10, valve closure member 20', and valve seat 30'. Valve closure member 20' is machined to have a curved surface or face in the form of an arc segment 21 of a circle of constant radius R.sub.20' extending from axis 105 (projecting out of the plane of the figure). Axis 105 is at the center of curvature of the arc face of valve closure member 20' and may, for example, be perpendicularly intersecting centerline axis 104 of fluid flow through port 10. Valve seat 30' may likewise be machined to have a sealing surface or perimeter face in the form of an arc segment 31 of a circle also of constant radius R.sub.20' less the interference required to seal the fluid under rated pressure. Thus, valve closure member 20' and valve seat 30' may have mating surfaces which extend coaxially from axis 105 to form adjacent concentric cylindrical circular arc segments.
The eccentric valve structure of FIG. 3 attempts to minimize the sliding together of valve closure member 20' and valve seat 30' until closure by pivoting or rotating valve closure member 20' about axis 108. As shown in FIG. 3, center of curvature axis 105 is offset laterally from axis 108 (exaggerated in the drawing). Also, center of curvature axis 105 follows an arc of circle 107 about rotational axis 108 as valve closure member 20' rotates. This can be readily seen by comparing the location of axis 105 with respect to transverse axis 109 in FIGS. 3-5. In FIG. 3, axis 105 is below axis 109; in FIG. 4, axis 105 intersects axis 109; and in FIG. 5, axis 105 is above axis 109.
Valve closure member 20', as mentioned above, is typically coated with elastomeric material 26 with the effective radius of the arc segment face of its sealing surface being equal to the value of R.sub.20'. This elastomeric material coating may be compressible or deflectible during closure so as to permit tight sealing.
As shown in FIG. 3, the fluid flow through port 10 has been reduced by initial engagement of leading edge 22 of valve closure member 20' with valve seat 30' by clockwise rotation about axis 108. At this position, elastomeric material 26 has not been significantly compressed by valve seat 30' anywhere around the periphery of port 10. Thus, the opposing arc segment faces of valve closure member 20' and valve seat 30' have different clearances around the periphery of the valve seat 30'. This prevents complete mating of the opposing faces. While there may be no significant clearance between leading edge 22 and valve seat 30', there will be clearance between trailing edge 24 and valve seat 30', and an appreciable gap for fluid leakage between these edges at the centerline of port 10, indicated as Point A, as well as at the trailing edge 24. Thus, all the sealing surfaces of valve closure member 20' and valve seat 30' do not meet substantially simultaneously upon initial contact of leading edge 22 and seat 30'.
FIG. 4 shows the valve arrangement of FIG. 3 as valve closure member 20' is further rotated clockwise about axis 108 to the normally closed position where valve closure member 20' and valve seat 30' are in continuous contact along the arc from leading edge 22 to trailing edge 24. At this position, the deflection and sealing interference by compression of elastomeric coating 26 on valve closure member 20' is substantially equal about port 10 along the surface perimeter of valve seat 30'. Thus, substantially uniform fluid sealing is achieved without any gaps along the valve seat.
However, additional clockwise rotation of valve closure member 20' about axis 108 will result in an overclosed situation, not uncommon in practice, as shown in FIG. 5. In this position, the maximum fluid sealing interference of valve closure member 20' and valve seat 30' by compression of elastomeric material 26 is at point A. Lesser sealing interference and compression of elastomeric material 26 is along trailing edge 24 of valve closure member 20'. Least interference and coating compression is along leading edge 22. This uneven compression and deformation often results in destructive stresses on valve closure member 20' and coating 26. Such stresses reduce the useful lifetime of the valve and decrease sealing integrity as well as increase the required torque. As these stresses result from the different circular curvatures of the sealing surfaces, they may also be found in eccentric valves of the type shown in the above-mentioned patent.
Although the closure problems mentioned above were described as resulting largely from the addition of a typical elastomeric coating on the valve closure member, similar problems also arise where the materials forming either the valve closure member, the valve seat, and/or portions of the valve housing are significantly compressible as well as where these elements are formed from metallic materials. Sealing surfaces that are still compressible even at complete closure are generally desirable where the fluid flow may contain entrained solids which may become trapped between seating surfaces.
It is therefore an object of the present invention to provide an improved valve means.
Another object of the invention is the provision of a valve closure member having reduced wear during extended operation.
A further object of the present invention is to provide improved fluid sealing during normal closure and overclosure of a valve closure member.
Still another object of the present invention is the provision of a valve closure member having equal contact stresses along sealing surfaces which can be achieved with maximum construction and assembly tolerances.
Yet, still another object of the present invention is to reduce the torque required to effectively seal the fluid within a valve.
These and other objects of the present invention are achieved in the provision of shaped or curved sealing surfaces, at least one of whose cross-sectional arc segment forms a portion of an involute, for the valve closure member and the valve seat of a rotary valve means for controlling fluid flow therethrough by rotation of the valve closure member to sealingly engage the valve seat. By machining the sealing surfaces of the valve closure member and of the valve seat to the form of an involute, these surfaces will engage substantially simultaneously over the entire perimeter of the valve seat during valve closure even if the valve closure member has an elastomeric coating or seating material. After initial contact and during overclosure, the compression and sealing of resilient seating material will likewise be uniform over the entire engaging surface and around the periphery of the valve seat.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings which show, for purposes of illustration and example only, preferred embodiments in accordance with the present invention.